1. Field of the Invention
The present invention relates generally to a circuit and method of controlling transient currents resulting from connecting a load to power conductors. More particularly, the present invention relates to a networked system in which the power conductors provide electrical power to a plurality of nodes. The inventive circuit and method control the turn-on and reset of a voltage-controlled switch which connects a load at one of the nodes to the power conductors, thereby limiting transient currents on the power conductors.
2. Description of the Related Art
Networked control systems typically include a number of device nodes coupled to a set of common conductors for transmitting power and data. The node devices often include both sensors and actuators of various types, as well as microprocessor-based controllers or other command circuitry. Moreover, some sensor and actuator nodes may also include signal processing capabilities, memory devices, and so forth. Power supplies coupled to the network furnish electrical energy via the network media to operate the sensors, actuators and other devices requiring an external power source. In operation, networked sensors provide information via the physical communications media relating to the states of various operating parameters. Other devices on the network process the transmitted parameter data and command operation of networked actuators, such as relays, valves, electric motors, and so forth. One device network of this type is commercially available from the Allen-Bradley Company of Milwaukee, Wis. under the commercial designation, DeviceNet.
Unlike unpowered data networks, powered industrial control networks pose unique problems for the transmission of both electrical power and data to and from networked devices. For example, the provision of power conductors and data conductors in a single cable can result in unwanted noise and other interference between the conductors, ultimately leading to bit errors in the transmission of the digitized data. A source of such interference or noise is transient current on the power conductors that is generated, for example, by connecting or disconnecting networked devices, energizing or deenergizing networked actuators, etc. Such noise sources must be controlled to reduce the likelihood that the generated noise may corrupt data transmitted to networked devices and, ultimately, the overall reliability of the network.
In addition to data corruption, noise sources may produce other undesirable effects. For example, due to the inherent impedance and reactance of the power conductors, power conductor current transients can result in power conductor voltage transients, which, in turn, can interrupt operation of networked devices or even potentially damage networked devices. Accordingly, control of such current transients is further desirable to ensure continued and reliable operation of networked devices.
There is a need, therefore, for a device and method to control current transients that may result in detrimental voltage transients on the power conductors. Such a device and method would be particularly desirable to prevent interruptions in the operation and damage to networked devices which all receive power via the power conductors. Further, in networked systems in which the power conductors and data conductors are provided in a single network media cable, it would be desirable to control such current transients to reduce the risk of bit errors that may be introduced on the data conductors as a result of transients on the power conductors.